This invention relates to a system and apparatus for the electrostatic atomization of liquids, particularly paint. The invention is particularly useful in assembly or conveyor line applications of paint, wherein articles move along such a line past stations designated for the application of paints in single or multiple colors.
In the assembly line manufacture of products, particularly products having large surface areas requiring paint, there has long been a need for automated systems for application of such paint, or other coating materials, in a consistent, high-quality manner with minimum loss of the paint or coating material. In older assembly line operations such products were typically painted using manually operated air spray paint guns in booths especially designed for the passage of the product along an assembly line, and for the convenience of personnel to paint the products as they pass through the booth. Since a large amount of paint applied in this manner is lost through overspray, such booths typically have a controlled air flow to direct overspray into residue collectors which are frequently or continuously cleaned to remove the residue from the booth. The oversprayed paint is considered a waste byproduct of the painting operation, and the residue collection facilities add energy and other costs which contribute to the overall plant operating expense.
More recent assembly line manufacturing techniques have utilized so-called "automatic" spray guns, either fixedly mounted or mounted on vertically reciprocable arms, which can be actuated at the precise moment the product passes in front of the paint station. Such systems are usually used in conjunction with manually operated spray guns to enable the complete coating of all surfaces, including surfaces of high curvature and unusual geometry which automatic spray equipment cannot cover. Automatic paint spray systems typically also suffer from a high percentage of loss of the coating material from overspray and other factors.
Automobile assembly plants typically use such systems for the painting of the body shell, fenders, hood and trunk lids, and other components of considerable surface area. Automobile assembly plants do not, for a variety of reasons, construct vehicles with any degree of color conformity, but rather utilize a system wherein an almost random sequence of colors may be applied to any given sequence of automobile bodies passing along the assembly line. This requirement imposes upon automobile assembly plants the additional requirement that automatic and manual paint spray equipment be equipped to apply any of a preselected number of colors in any order, and to change from one color to another in the brief period of time between the passing of one product and the arrival of the next product on the moving assembly line. Color changers have been adapted to accomplish this function, and today automobiles roll off the assembly line in any variety of sequential color combinations.
Electrostatic paint spraying has long been recognized as providing an improvement in coating efficiency over conventional spray painting techniques. The use of electrostatic paint spray guns causes an electrostatic field to develop between the spray gun and the article to be painted, the field both assists in the atomization of the paint and also develops attractive forces between the spray gun and the article to be painted in such a manner so as to attract paint particles to the surface to be coated. Electrostatic paint spraying techniques yield a considerably higher efficiency of coating over conventional air spray or conventional airless spray techniques. Applications of up to 95 percent of the paint material emitted from an electrostatic spray gun are readily applied to the product, resulting in very low paint losses due to overspray and other factors.
Centrifugal atomization of paint, accomplished by applying an atomizing paint through the use of a rotating disc, usually in combination with electrostatic forces, enables improvements in both coating quality and in coating deposition efficiency. However, until fairly recently, such techniques have not been utilized in the automobile industry, both for the reason that a special equipment had not been developed for this purpose, and also for the reason that centrifugal atomizers generally tended to provide a poorer quality of coating. However, recent improvements in centrifugal atomizer technology have improved coating quality, and new inventions, such as the present invention, are being developed to enable the adaptation of centrifugal atomizers to assembly line paint spraying operations involving large products such as automobiles.